1. Field of the Invention
The present invention relates generally to honeycomb structures. More particularly, the present invention relates to providing ventilation of honeycomb structures which are flexible.
2. Description of Related Art
Honeycomb structures have found wide use in many settings where high strength and light weight are required. Many honeycomb structures are in the form of panels which are made up of honeycomb that is sandwiched between two side surface sheets. A common honeycomb configuration is the one in which honeycomb walls are interconnected to form hexagonal cells. Hexagonal honeycomb sandwich panels are strong and rigid structures. Panels utilizing hexagonal honeycomb cores have been used extensively in aircraft and spacecraft where relatively planar structural elements are needed which are lightweight and strong.
In many situations, it is desirable to provide honeycomb sandwich panels which have the strength associated with conventional hexagonal honeycomb, but which also are sufficiently flexible to be formed into nonplanar shapes. Exemplary flexible and formable honeycombs are described in U.S. Pat. Nos. 3,227,600 and 3,342,666. These types of flexible honeycombs can be formed into structures having compound curves. Circular structures with relatively tight radii may also be formed. Common types of flexible honeycombs are available from Hexcel Corporation under the trademark FLEX-CORE.RTM. and DOUBLE-FLEX.TM..
Honeycomb structures become closed cellular systems when solid side surface sheets are added to form the final honeycomb panel. These closed systems are made up of a multitude of interconnected closed cells. It is essential in certain circumstances that the cells of the honeycomb panel be vented amongst themselves and also vented to the panel exterior. For example, aircraft and space vehicles are subjected to large changes in air pressure. Honeycomb panels must be vented in order to avoid the build-up of damaging pressures within the honeycomb. In addition, there are certain situations where the honeycomb panel is designed to include discreet passageways or conduits through which various gas or vapors are transported. The selective venting of honeycomb structural panels to form such conduits passing through the panels is especially useful on spacecraft where multipurpose elements are desirable.
Some honeycomb manufacturing processes involve the heating and/or generation of gases during final formation of the honeycomb panel. In these situations, the honeycomb core must also be vented or made "breathable" in order to avoid excessive build-up of pressure within the individual cells.
Conventional honeycomb has been vented in a variety of ways. Venting configurations depend in large part upon the final intended use for the sandwich panel assembly. In those situations where structural strength is a prime consideration, venting configurations typically involve providing one or more small vent holes in each cell wall. Alternatively, when strength could be sacrificed in favor of lower densities and high vent rates, honeycomb cores have been made utilizing perforated materials which provide numerous permeations in the honeycomb through which venting can occur.
There are significant structural differences between conventional non-flexible honeycomb and flexible honeycomb. In addition, there are significant differences in the processes by which these different types of honeycombs are manufactured. As a result, the venting procedures and configurations which typically have been utilized for the more conventional honeycomb cores are not applicable to flexible honeycomb core such as FLEX-CORE.RTM. and DOUBLE-FLEX.TM.. Although perforated material may be utilized to provide venting of flexible honeycomb core material, such perforated materials are not well-suited for those situations where high strength or selective passage of certain media through the honeycomb is desired. Accordingly, there is a present need to provide a simple, effective and economically efficient process for making vented flexible honeycomb sandwich panels.